Historically, celluloid was often seen as the first plastic matter of industrial relevance. Contemporary to the fast industrialization of late 19th century and early 20th century, it rapidly became a workhorse material for commodity products. Its origin and production are found in the following patents:
Charles E. When, “Method of Shaping Pyroxylin, Celluloid, And Like Materials”, U.S. Pat. No. 1,461,299, dated Jul. 10, 1923.
William G. Lindsay, “Process of Making Sheets of Pyroxylin Compound” U.S. Pat. No. 1,468,820, dated Sep. 25, 1923. The ancestor of celluloid, Parkesine, was developed by an Englishman named Alexander Parkes in 1862. He discovered that a mixture of cellulose treated with nitric acid and different solvents had thermoforming abilities. Unfortunately, the lack of plasticizer in its formulation resulted in a material that would crack over time making high scale production impossible. This was until John Wesley Hyatt discovered that camphor was a natural solvent for nitrocellulose making the material the first thermoplastic produce on an industrial level (see U.S. Pat. No. 88,663, dated Apr. 6, 1869, antedated Mar. 25, 1869). Hyatt decided to name his disclosure “celluloid”.
Due to its thermoforming and combustion characteristics, celluloid is a very interesting product for the armament industries. Mortar increment made of celluloid sheets has been used as a container for propellants.
There are two processes currently used in order to produce celluloid sheets on an industrial scale: The block method, in which a uniform block of celluloid is cut into thin individual sheets, and the lacquer method in which the celluloid is dissolved in lacquer, spread on a conveyor belt and dried.
As explained above, the bloc method consists of making a block of celluloid followed by cutting into sheets. Typically, there are six main steps: mixing using a sigma blade mixer, plastifying using a “roll-mill” apparatus, blocking, cutting, drying and final shaping.
Using a sigma blade mixer, the nitrocellulose and camphor are mixed together in the presence of ethanol and acetone is added in order to be able to mix the dough thoroughly and ensure no undissolved nitrocellulose remains. The mixed dough is split in fractions and moved to a “two roll-mill” heated where every fraction is processed individually using a two roll mill and piled on top of each other to form a thick celluloid carpet. The thick carpet is moved to a jacketed heated press. Heat and pressure are applied on the material for between 8 and 12 hours typically. The resulting block is cooled down and oscillated back and forth on a hydraulic table equipped with a knife. The knife cuts a thin slice of the block each time as the block is passing underneath.
In order to reach the correct solvent ratio the sheets are suspended on a rack and dried in a room using hot air blowers. The sheets are piled on top of each other and placed in a multiple deck press where they are heated for 40 minutes to soften the celluloid sheets and make sure that they come out straight and flat. An optional cutting step is performed to give the final dimensions to the sheet.
The block method has several disadvantages, primary among them is the fact that it can only be carried out as a batch process. This results in slower production and higher costs.
The lacquer method is a method of celluloid production that is performed by first dissolving the raw material (nitrocellulose, camphor, optional stabilizer, etc.) in solvent and pouring the solution in a continuous strip on a conveyor belt where the solvent is removed leaving a clear continuous sheet of celluloid at the end of the conveyor.
This method is a continuous process and is inherently more efficient than the block method above. However, the lacquer method also bears several disadvantages including: the thicknesses of the sheets are less consistent than the block method. Depending on the thickness of the celluloid sheets, there will be residual solvents in the final product which can result in end-user processability problems. In addition, the amount of solvent that needs to be recovered is much higher than with the block method and if a foaming agent is required, the uniformity of the foaming agent concentration in the celluloid lacquer cannot be predicted.
U.S. Pat. No. 4,120,920 describes and teaches a process for the production of extrusions formed of compositions based on plasticized nitrocellulose and a method of continuous production of propellant extrusions based on nitrocellulose gelled with an explosive oil and with a screw-extruder for carrying out such a method.
U.S. Pat. No. 1,979,762 describes and teaches batch compounding process for preparing pyroxylin (nitrocellulose) sheets where the pyroxylin is in the form of a colloidal gel of bread-dough stiffness and extruded through an orifice and formed into a soft web containing a considerable amount of solvent.
United States Patent Application Publication No. 2008/0242794 describes and teaches a polymer composition, including celluloid, including a melt-processed polymer compounded with a colour stabilizer and a silver-based antimicrobial agent.
U.S. Pat. No. 4,608,210 describes and teaches a method for producing plastically bonded propulsion powders or explosives by means of an extruder consisting of two co-rotating or counter-rotating screw shafts and a forming head.
US Patent Application Publication No. 2002/0079031 describes and teaches a methodology where solids including acrylic gum/guanidine nitrate, oxidizing filler and additives, and liquids are separately introduced into a twin-screw mixer-extruder, conveyed and kneaded to form a paste, degassed and extruded in the form of rods.
U.S. Pat. No. 2,171,095 describes and teaches an extrusion apparatus used for extruding plastics such as pyroxylin (nitrocellulose) into solid block forms which are subsequently adapted to provide patterns, designs and colour effects.
Thus there exists a need for a continuous extrusion process for preparing cost-effective celluloid of various geometries having uniform properties. Further, other desirable features and characteristics of the inventive subject matter will become apparent from the subsequent detailed description of the inventive subject matter and the appended claims, taken in conjunction with the accompanying drawings and this background of the inventive subject matter.